What is this yellow mucus stuff in the corner of my eyes?

Mucus. The usually light yellow substance that collects in the corners of the eyes is mucus. It's made by the membrane that surrounds your eye. It also has moisture and oil secretions associated with it. If your eyes are otherwise healthy feeling, then not to worry. If they are dry, scratchy, allergic etc, then your eye may make much more, or secrete purulent material that is copious and very sticky.
Mucous. Can be from dryness or irritation. In the corner of the eye it is of no concern.